1. Field of the Invention
The present invention relates to a process for producing methacrylic acid or an ester thereof from isobutyric acid or an ester thereof.
2. Description of the Prior Art
It is known that methacrylic acid is formed when isobutyric acid undergoes oxidative dehydrogenation with molecular oxygen in the gas phase.
A catalyst containing molybdenum, vanadium, and phosphorus is effective for this reaction. Particularly effective is a heteropoly acid composed of molybdenum and vanadium for the condensation ligand atoms and phosphorus for the central atom. Such a catalyst exhibits high activity and high selectivity. (Japanese Patent Laid-open No. 78120/1973)
It is known that the catalyst is more improved in its performance by adding copper (Japanese Patent Laid-open No. 31018/1977), chromium (Japanese Patent Laid-open No. 105113/1977), lithium (Japanese Patent Laid-open No. 98131/1980), lead (Japanese Patent Laid-open No. 15238/1981), cadmium, uranium, indium, lanthanum, and cerium (Japanese Patent Laid-open No. 105112/1977), tellurium (Japanese Patent Laid-open No. 4016/1975), and thallium (Japanese Patent Laid-open No. 4017/1975). These catalysts are superior in activity and selectivity at beginning, but decrease in activity after use for a long period of time. Therefore, they are not satisfactory for industrial use.
It is known that a carbonyl compound, carboxylic acid, or ester having an isopropyl group at the .alpha.-position undergoes selective oxidative dehydrogenation with a heteropoly acid catalyst. The catalyst can be improved in activity and selectivity by adding a second component. The kind and quantity of the adequate second component vary depending on the type of the substrate.
Japanese Patent Laid-open No. 100324/1980 discloses catalysts of Mo-V-P-X-Y-O (where X is Cu, Sn, Th, Al, Ge, Ni, Fe, Co, Zn, Ti, Pb, Re, Zr, Ce, W, Bi, and As; and Y is K, Rb, Cs, and Tl) for producing methacrolein and methacrylic acid by oxidative dehydrogenation of isobutyraldehyde. Also, Japanese Patent Laid-open No. 100325/1980 discloses catalysts of Mo-V-P-Cu-X-Y-O (where X is As, Th, Al, Ge, Ni, Fe, Co, Zn, Ti, Pb, Re, Zr, Ce, W, Bi, Sn, and Cr; and Y is K, Rb, Cs, and Tl) for producing methacrolein and methacrylic acid from isobutyraldehyde in the same way.
However, in the case where the reaction substrate is isobutyric acid or an ester thereof, the effective additive is not the same; particularly, heavy alkalis such as K, Rb, and Cs greatly decrease the oxidative dehydrogenation and therefore are unfavorable for reaction.
Therefore, there is a continuing need for the catalyst having high activity, selectivity and long life.